This invention relates generally to thermal conductivity detectors used in gas chromatography and more particularly to bridge type detectors.
Gas chromatographic separating columns utilize thermal conductivity detectors to determine the components of gas flowing therethrough. These detectors normally include a bridge circuit, driven by a constant-current source, which employs temperature-sensitive, heated filaments or thermistors having a positive temperature coefficient of resistance. One of these, or a pair of them, are placed in opposite legs of the bridge and are immersed in a reference or carrier gas flow of known thermal conductivity. A second one or pair, are placed in the remaining, opposite legs of the bridge and immersed in the gas flow at a point in the column after the introduction of the sample. The bridge output voltage is then dependent upon the difference in thermal conductivities between the reference gas and the sample gas to be measured, and the housing of the detector.
E.g., if the reference or carrier gas has a high thermal conductivity and the gas to be sampled a lower one, the latter causes the corresponding heated filaments to increase in temperature (and accordingly electrical resistance) relative to the reference filaments resulting in a bridge unbalance. This produces a proportional bridge output voltage which can be measured and recorded.
Although the sensitivity of the thermal conductivity detector increases with an increase in filament temperature, if it should rise too high, the filament will burn out.
One technique for preventing such overheating of the filament is disclosed in German AS No. 1,523,011. The supply voltage across the bridge circuit is tapped and compared with an adjustable reference voltage.
If the tapped voltage exceeds a predetermined limit, an overload protective circuit is activated which in turn controls the current output of the constant current supply so as to reduce the bridge circuit supply.
Another arrangement is disclosed in German OS No. 1,648,276. Here the measuring bridge is placed in the leg of another bridge circuit. The latter which is driven by a constant current source contains fixed resistors in the remaining legs thereof. When the bridge output voltage exceeds a given limit, corresponding to a given resistance change in the measuring bridge, an overload protective circuit again operates upon the constant current source to reduce the supply current fed to the bridge circuit.
Yet, another known technique employs a measuring bridge circuit, again, placed in one leg of a second bridge circuit. An adjustable resistor is placed in the opposite leg. By setting this resistor at a particular value representing a particular temperature for the filaments a balance point for the bridge is determined which occurs when the total resistance between the supply terminals of the measuring bridge equals the preset adjustable resistor.
The resistance of the measuring bridge depends on the heating of the filaments which in turn is a function of the current supplied thereto. When the bridge is unbalanced, the output voltage of the latter is supplied to the input of a differential amplifier. The output of the amplifier drives the base of a current control transistor which is connected between a power supply and the bridge supply point. The bridge supply current is thus varied to effect the necessary change in the measuring bridge resistance to balance the second bridge circuit.
With such an arrangement, the temperature of the sensing filaments can easily be preset and adapted to the varying requirements of individual applications. However, this arrangement has also been found to result in certain non-linearities in the measuring bridge circuit due to the fact that its resistance also changes when the filaments are exposed to the sample gas components. This causes an unbalance of the other bridge circuit which results in an altering of the current supplied. As a consequence, the measuring bridge circuit is accordingly affected with different currents depending on its own degree of unbalance.
It is therefore a primary object of this invention to provide a thermal conductivity detector circuit which will allow for presetting the filament temperature and still provide a linear reading when the measuring bridge becomes unbalanced due to the influence of the sample gas components on the filaments.